The present invention relates generally to the field of curtain coating, and in particular to a new and useful apparatus and method for measuring the runnability of a coating process.
Curtain coating machines are used to create uniform coatings on a particular substrate. Prior techniques for curtain coating generally experience a large loss of coating material as a result of undue experimentation while attempting to achieve uniformity in the coating material. In particular, prior techniques for curtain coating have been unable to focus on the crucial fluid properties which dictate runnability and have instead used time-consuming and expensive trial and error techniques.
It is common for new coating formulas to be modified and operating conditions adjusted to achieve good appearance or uniform coating lay at full commercial conditions. This is done by trial and error with expensive commercial and pilot machine trials. It is expected that measurements taken according to the present invention will eliminate that practice as the important properties of the coating can be measured and adjusted prior to a run. It is common for formulas to be modified over the span of several months. Several (e.g., 8 to 10) runs are performed under programs costing $80,000 to $100,000. It is expected according to the invention that this expense will be avoided by measuring important properties of the coating material using beaker quantities of the material in the lab. With the coating formula properly adjusted the trial effort will be considerably reduced and good results achieved on the first run.
Development of a suitable curtain coating is important, with many companies spending large sums of money to formulate such coating material. The material must lay down in a perfectly uniform coating thickness, and this is possible with curtain coating. Perfect uniformity has made curtain coating the coating technique of choice for photographic and other imaging paper applications, because the layer of coating is developed to a solid color and any non-uniformity can be seen easily. This coating method has high process efficiencies with little operating adjustments required. The uniformity advantages combined with improved efficiency make it very interesting for a broad range for coated products.
Various methods are known for measuring fluid properties. U.S. Pat. No. 5,590,560, for example, teaches an apparatus and method for measuring dynamic viscosity, surface tension, and dilational elasticity. Electrostatic forces are used to distort a film rather than gravity and an air jet as with the present invention. The apparatus of U.S. Pat. No. 5,590,560 is measuring surface dilatational elasticity and not the bulk properties of dilitation index and bulk elasticity. The time for full distortion is 0.1 seconds as taught by the patent. The invention achieves distortion in 0.8 milliseconds. The prior patented apparatus is measuring the surface distortion force under compression whereas the invention is measuring under stretching. The prior apparatus is not measuring the same thing as the invention.
U.S. Pat. No. 5,590,560 teaches distorting a film of coating to twice its thickness. So, if one thinks of the motion of something flat and uniform being pulled up into a sine wave, it is the opposite to what the invention is testing. The fluid is bunching up, not thinning out. The volume/diameter is getting smaller, whereas in the inventive test, the volume/diameter is getting larger.
U.S. Pat. No. 5,792,941 discloses a method for determining dynamic physical properties, such as surface tension and viscosity. A base is provided with an open channel having a predetermined cross-section and height, and a fixed distance is identified within the channel. The liquid to be tested is deposited in the channel and a measurement is taken of the time that the liquid requires to flow through the fixed distance.
U.S. Pat. No. 6,185,989, for example, teaches a device for dynamic measurement of the surface tension of a liquid by a bubble pressure process. Gas pressure is used to form a boundary between liquid and gaseous surface, and an electrically operated device may then measure the pressure inside the bubble.
One curtain coating method has been developed for measuring static and dynamic surface tension, but has failed to address other fluid properties of a coating that are relevant for measuring and improving runnability of a coating. U.S. Pat. No. 5,304,402 claims a curtain coating method in which a solution injector, having at each of two side ends an edge guide, applies a coating at a flow rate of 2 cc/(cm*sec). This prior method is able to achieve flow rates as low as 2 cc/(cm*sec) by restricting the horizontal movement of a coating solution in the freely falling coating film by adjusting the width and the radius of the solution contacting surface of the edge guide based on the measurements of static and dynamic surface tension.
The following articles and patents disclose various curtain coating techniques and characteristics:
1. D. R. Brown, A Study of the Behaviour of a Thin Sheet of Moving Fluid, Journal of Fluid Mechanics, Vol. 10, part 2, pages 297-305;
2. W. Van Abbeyenyen, W. Mues, B. Goetmaeckers; Agfa-Gevaert in an unpublished presentation at an AlChE meeting circa 1990;
3. Summers et al., The Kinematics and Dynamics of Free Surface Coating Fluids, Final Report on EPSRC Grant GR/L17191, June 2000; and
4. U.S. Pat. Nos. 4,216,290; 5,824,887; and 5,962,075.
The following additional patents disclose fluid testing techniques: U.S. Pat. Nos. 4,196,615; 4,646,562; and 5,479,816.
Although several examples have been given of methods for determining fluid properties, relevant to the runnability of a coating, no known method addresses measurement of each of the fluid properties that are necessary for predicting the runnability of a coating for a curtain coating machine.
It is an object of the present invention to provide an apparatus and method for determining the runnability of a coating based on fluid property measurements of the coating. The term xe2x80x9crunnabilityxe2x80x9d is used here to mean the characteristics of a coating material that gives reasonably satisfactory results when used to coat a substrate during a curtain coating run.
It is also an object of the present invention to provide an apparatus and method for measuring a coating""s resistance to stretch under free fall conditions. The resistance to stretch by a fluid is caused by a combination of the fluid""s surface tension, viscosity, elasticity, and dilatant behavior. The central property measured is a new property called xe2x80x9cstretch,xe2x80x9d which is actually a combination of surface tension and viscosity. A stretch number is derived from the minimum flow rate of a falling stream that is forced out of a small round tube with more flow rate than is required for momentum to exceed contracting forces of surface tension. Actual surface tension is measured if the viscosity is less than one centipoise or otherwise negligible.
It is another object of the present invention to provide an apparatus and method for determining the landing velocity of a coating when it makes contact with a moving sheet of substrate. The landing velocity can be measured from a combination of the stretch, acceleration, and viscosity.
Undesirable elasticity and dilatant behavior impede a fluid from stretching out and can be detected by measuring the acceleration of a falling stream. Therefore, another object of the present invention is to provide an apparatus and method for measuring acceleration of a coating as it is observed falling between two points assuming a uniform accelerating force. The assumed accelerating force is the force of gravity, minus surface tension, elasticity, dilatant behavior, and viscosity, expressed in ft/sec/sec. Dilatant behavior refers to particle interactions that retard fluid flow, such as particles jamming against each other in the fluid. Coatings for printing paper, for example, contain two micron diameter pigment particles which can flock together into six to eight micron clusters.
Accordingly, an apparatus and method are provided for measuring various fluid properties, such as surface tension, viscosity, elasticity, and dilatant behavior, to predict the performance of the coating to be applied with a curtain coater, for testing and/or improving the runnability of the coating.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.